Vapor compression distillation process



Nov. 5, 1963 w. s. NATHAN 3,109,782

VAPOR COMPRESSION DISTILLATION PROCESS Filed Dec. 20, 1955 //VV5/v we W/L FEEO 522/7051. New/my A fr ENE r5 United States Patent 3,109,782 VAPOR COMPRESSION DISTILLATION PROCESS Wilfred Samuel Nathan, Sunbury-on-Thames, England, assignor to The British Petroleum Company Limited, London, England, a British joint-stock corporation Filed Dec. 20, 1955, Ser. No. 554,318 Claims priority, application Great Britain Dec. 30, 1954 5 Claims. (Cl. 202-75) This invention relates to an improved method for use in evaporator and/ or distillation systems.

In distillation systems wherein a vapour phase product is removed overhead from a distillation unit and thereafter condensed by indirect heat exchange with a heat exchange medium, there is involved a loss to the system of thermal energy equivalent to the latent heat of condensation of the overhead product. While it is sometimes possible to reduce this loss by heat exchanging the overhead product with cold feedstock to the distillation unit, the feedstock and overhead product temperatures are not always so related as topermit recovery in this manner of more than a low proportion of the available heat of the overhead fraction.

It is known that by mechanical compression of the overhead vapour stream under conditions such that its temperature is raised above the temperature of the distillation column base and its pressure is increased to a pressure at which said overhead stream condenses at the column base temperature it is possible to recover the latent heat of condensation of the overhead product by indirect heat exchange in a column base reboiler. This method, While admittedly of high thermal efficiency, has the disadvantage that in relation to the costs of a distillation column of given throughput, the costs attending the installation and maintenance of a mechanical vapour compressor of the necessary size are undesirably high. It is therefore among the objects of this invention to provide a heat recovery system which, while dispensing with mechanical compressors, still provides good thermal efiiciency and hence overall economic operating conditions.

It is another object of the invention to provide an improved method for obtaining heat from a vapour phase stream. Other objects will appear hereinafter.

According to the present invention there is provided a process which comprises distilling or evaporating a volatile liquid in a still, with recovery of a vapour phase fraction, dividing said traction into a first stream and a second stream, condensing said first stream, pumping the condensate through a heater wherein said condensate is vaporised to yield heated vapour at a temperature and pressure respectively above the temperature and pressure or said vapour phase fraction and thereafter blending said heated vapour with said second stream, under conditions such that the back pressure exerted by the blend or by said heated vapour upon said second stream is insuflicient to stop or reverse the flow of said second stream, and condensing said blend by indirect heat exchange with a stream of colder fluid medium being fed to said still.

Preferably the stream to be fed to said still is a liquid reboiler stream taken from the base of said still.

Furthermore the temperature diilerence between the base of said still and said vapour phase fraction should be small, preferably less than 5 C.

In general, part of the vapour phase fraction after heat exchange with a reiboiler stream, will be returned to the still as reflux preferably after passing the reflux through a further heat exchanger in which its temperature is adjusted to the required still temperature before being returned to the still.

Preferably in any of the above embodiments, the heated vapour is blended with said second stream by employing said heated vapour as the high pressure feed to a Venturi type injector to which said second stream is drawn as a low pressure stream.

Also according to the present invention there is provided an apparatus for obtaining from a condensable vapour its latent heat of condensation. According to the invention there is provided a condenser capable of receiving and condensing part of a continuous stream of said condensalble vapour, a pump adapted to pass condensate at increased pressure to a heater, said heater being adapted to convert said condensate to heated vapour at a temperature above the temperature of said continuous stream ocE condensable vapour, a Venturi type injector adapted to blend said heated vapour with a further part of said continuous stream of condensable vapour and a heat exchanger connected to said injector and adapted to pass the blend in indirect heat exchange with a colder fluid medium.

Preferably the Venturi type injector is a multi-stage injector.

Also according to the present invention there is provided a distillation or evaporator system comprising an apparatus as described above for obtaining from a condensable vapour its latent heat of condensation, and a distillation or evaporator still adapted to deliver a continuous stream of condensable vapour.

Preferably the still reboiler unit consists of the heat exchanger of the apparatus described above.

The term liquids is used herein with reference to substances which are in the liquid phase under the conditions of operating the still.

The invention is further illustrated but in no way limited with reference to the drawing accompanying the specification which is a flow diagram of a distillation system in accordance with the present invention.

With reference to said drawing:

Feedstock is introduced by line 1 to column 2 from which an overhead fraction is Withdrawn by line 3. Part of the overhead vapours are condensed in heat exchanger 13 and pass to surge tank 14, from which condensate is pumped by pump 15 to heater 1-6. From heater 16, heated and pressurised vapours are passed to injector 17 through which is drawn the remainder of the overhead vapours via line 18. The blended vapours are passed by line 19 to heat exchanger 11 in which a substantial proportion are condensed. The temperature of the overhead fraction is thereafter adjusted to the required distillation recycle temperature in heat exchanger 4 and passed to surge tank 5. Product is withdrawn by line 7, the re mainder of the overhead fraction being recycled to the column head by line 6. The heat exchanger receives a distillation reboiler stream by line 8, the rob-oiled fraction being returned by line 9'. A bottoms fraction is withdrawn by line =10.

The method hereinbefore described is, in general, particularly suitable for the treatment of any feedstock by distillation or evaporation wherein the temperature difference between column head and column base is small.

The method or this invention is especially suitable for removing a solvent from a less volatile material, as for instance in solvent refining, solvent dewaxing and solvent deasphalting, and also has applications in the processing of mineral oil, for example petroleum and its products, alcohols and essential oils.

The method hereinbefore described is also suitable for application to an evaporation system, for example, for concentration of solids in solution and such methods also lie within the scope of this invention.

I claim:

1. A process for the distillation of volatile liquids,

which comprises: "distilling a volatile liquid in a still, with recovery of a vapour phase fraction; dividing said fraction into a first stream and a second stream; condensing saidfirst stream; pumping the condensate through a a said second stream, under conditions such that the back pressure upon said second stream is insufficient to prevent the forward flow of said second stream; condensing said blend by indirect heat exchange with a stream of colder fluid medium being fed to said still; and, returning apart of said condensed blend to said still as reflux.

2. A processaccording to claim 1, wherein said reflux is passed through a further heat exchanger wherein its temperature is adjusted to the required still temperature before being returned to the still.

3. A process according to claim 1, wherein said stream to be tied to said still is a liquid reboiler stream taken 2 from the base of said still.

4. A process according to claim 1, wherein said heated vapour is blended with said second stream by employing said heated vapour as the high pressure feed to a Venturi type injector to which said second stream is drawn as a low pressure stream.

5. A process according to claim 3, wherein the temperature difference between the base of said still and said vapour phase fraction is less than 5 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,213,596 Baufre Jan. 23, 1917 2,127,004 Nelson Aug. 16, 1938 2,324,663 Aiton July 20, 1943 2,338,595 Packie Jan. 4, 1944 2,637,684 Buffurn May 5, 1953 FOREIGN PATENTS 9,815 Great Britain of 1889 182,711 Switzerland iFeb. 29, 1936 614,309 Great Britain Dec. 14, 1948 

1. A PROCESS FOR THE DISTILLATION OF VOLATILE LIQUIDS, WHICH COMPRISES: DISTILLING A VOLATILE LIQUID IN A STILL, WITH RECOVERY OF A VAPOUR PHASE FRACTION; DIVIDING SAID FRACTION INTO A FIRST STREAM AND A SECOND STREAM; CONDENSING SAID FIRST STREAM; PUMPING THE CONDENSATE THROUGH A HEATER WHEREIN SAID CONDENSATE IS VAPOURISED TO YIELD HEATED VAPOUR AT A TEMPERATURE AND PRESSURE RESPECTIVELY ABOVE THE TEMPERATURE AND PRESSURE OF SAID VAPOUR PHASE FRACTION AND THEREAFTER BLENDING SAID HEATED VAPOUR WITH SAID SECOND STREAM, UNDER CONDITIONS SUCH THAT THE BACK PRESSURE UPON SAID SECOND STREAM IS INSUFFICIENT TO PREVENT THE FORWARD FLOW OF SAID SECOND STREAM; CONDENSING BLEND BY INDIRECT HEAT EXCHANGE WITH A STREAM OF COLDER FLUID MEDIUM BEING FED TO SAID STILL; AND, RETURNING A PART OF SAID CONDENSED BLEND TO SAID STILL AS REFLUX. 